A long series of explosions has rocked Libya's capital in what could be one of NATO's most intense daytime bombing missions over Tripoli.

Witnesses said they have seen smoke in the area of Libyan leader Moammar Gadhafi's sprawling compound in the city.

As NATO continues its air campaign, world powers are increasing pressure on Gadhafi's government.

On Tuesday, the European Union announced plans to add six Libyan ports that are under the government's control to its sanctions list.

Meanwhile, Russian envoy Mikhail Margelov met Tuesday with members of the opposition Transitional National Council in the rebel stronghold, Benghazi. He said Moscow is interested in Libya being a sovereign and democratic country that is a "worthy member of the international community."

Also, China's Foreign Ministry said one of its diplomats based in Egypt recently held talks with the Libyan rebel council. The ministry also said Tuesday that Libya's foreign minister is on a three-day visit to Beijing.

China and Russia both abstained when the U.N. Security Council voted in March to establish a no-fly zone over Libya, and have called for a negotiated solution to the conflict.

In Brussels, NATO Secretary-General Anders Fogh Rasmussen said he would use a defense ministers' meeting Wednesday to repeat calls for the alliance to step up involvement in the Libya operation.

On Monday, Libyan rebels said they had taken control of the western mountain town of Yafran from forces loyal to Gadhafi after NATO airstrikes last week destroyed key government military targets, enabling opposition forces to advance.

Ethnic Berber fighters, who have joined the rebellion, retook Yafran, about 100 kilometers southwest of Tripoli. Pro-Gadhafi forces had attacked the western mountain region after local Berbers rose up against government troops at the beginning of the uprising.